In a recent system for transmitting a data signal at a high speed, a serial signal transmission system such as IEEE1394, Universal Serial Bus, Ethernet (registered trademark), or the like is often used. Each of those transmission systems includes: a serializer IC for converting parallel signals into a serial signal; a transmission line for transmitting the serial signal which is converted by the serializer IC to a deserializer IC; and the deserializer IC for receiving the serial signal transmitted from the transmission line and converting it into the parallel signals again.
In those serial signal transmission systems, in the case of using the serializer IC having input terminals of n bits, the parallel signals which are input to the serializer IC are not always limited to a parallel signal of n bits. In the case where the input signals are parallel signals of mbits (m is an integer smaller than n), the input terminals corresponding to (n−m) bits are unused redundant input terminals. Such redundant input terminals are liable to be influenced by noises because they are in a floating state and this results in an erroneous operation in the serializer IC.
Therefore, wirings or a pattern of a stable voltage level such as power source or ground is connected to the redundant input terminals through resistors of a low or high resistance. By such a construction, it is prevented that the redundant input terminals enter the floating state, thereby suppressing exogenous noises.
However, in the case of such a serial signal transmission system, although the exogenous noises can be suppressed, radiation noises which are caused by the transmission of a high-speed digital signal cannot be suppressed. That is, the more the number of times of signal transition per unit time of the digital signal which is transmitted increases, a harmonic component of the serial signal increases and radiation noises are generated.
In the serial signal transmission system according to IEEE1394, a technique for encoding the serial signal as a preprocessing of the serial signal which is transmitted has been proposed in Japanese Patent Application Laid-Open No. 2001-036590 (Patent Document 1). That is, such a technique that when the input parallel signals are converted into a serial signal, the parallel signals are rearranged according to a predetermined rule by using a logic circuit and encoded, thereby suppressing the number of times of transition of the signals has been disclosed.
However, in the serial signal transmission system disclosed in Patent Document 1, it is necessary to execute the encoding process by a logic circuit on the parallel signals before they are input to the serializer IC. Therefore, it is necessary to add a special circuit other than the serializer IC to the encoding process, its costs are very high, and such a construction cannot be simply applied to the general all-purpose serializer IC.
In the case of the foregoing serial signal transmission system in which the wirings or pattern of the stable voltage level is connected to the redundant input terminals, the serializer IC rearranges the input parallel signals of n bits in predetermined order and transmits them as a serial signal of a bit pattern which are temporally continuous. Therefore, the bit signals from the redundant input terminals are also converted into a serial signal. That is, a signal which is always fixed to “1” if the redundant input terminals are connected to the power source and is always fixed to “0” if the redundant input terminals are connected to the ground is transmitted.
Therefore, since the bit signals from the redundant input terminals mentioned above are constant, a transition of the signal occurs between the constant signal and each of the bit signals before and after this signal at a probability of ½. Such a signal transition is fundamentally unnecessary in the serial signal transmission. If the signal transition is eliminated, the generated radiation noises can be suppressed without exerting an influence on the serial signal transmission.